Memory devices are used in integrated circuitry to store information in the form of binary data. There are various types of memory devices including volatile semiconductor memory in which stored data is retained as long as power to the system is not turned off, such as dynamic random access memories (DRAMs). Non-volatile memories, such as read-only memories (ROMs), retain stored data even when power is discontinued, but have less storage capability and programming options than volatile memories. Although there are non-volatile memories such as programmable read-only memory (PROMs) and electrically-erasable PROM (EEPROMs) that permit limited reprogramming, there are limits on the programming capacity of such memory devices.
Programmable metallization cells (PMCs) comprise a fast ion conductor or resistant variable material, typically a chalcogenide material having metal ions therein, which is disposed between two electrodes comprising an electrically conducting material (e.g., an anode and a cathode), as described, for example, in U.S. Pat. No. 6,084,796 (Kozicki et al., Axon Technologies). Resistant variable materials (or fast ion conductors) are capable of assuming a high resistance “off” and a low resistance “on” state in response to a stimulus for a binary memory, and multiple generally stable states in response to a stimulus for a higher order memory. The resulting memory element is non-volatile in that it will maintain the integrity of the information stored by the memory cell without the need for periodic refresh signals, and the data integrity of the information stored by these memory cells is not lost when power is removed from the device.
The resistant variable material (e.g., chalcogenide-metal ion material) undergoes a chemical and structural change at a certain applied voltage. Specifically, at a threshold voltage, plating of metal from metal ions occurs. A metal dendrite grows within the chalcogenide-metal ion material, eventually connecting the two electrodes. The growth rate of the dendrite is a function of the applied voltage and time. The growth of the dendrite can be stopped by removing the voltage or the dendrite can be retracted back towards the cathode by reversing the voltage polarity at the anode and cathode.
Changes in the length of the dendrite affect the resistance and capacitance of the PMC. If dendrite growth is continued until it effectively interconnects the electrodes to electrically short them together, a drop in the resistance of the resistance variable material results. The resistance variable material can be returned to a highly resistive state by reversing the voltage potential between the anode and cathode, whereupon the dendrite is disrupted. Thus, such a device can function as a programmable memory cell of a memory circuitry.
An exemplary resistance variable material comprises germanium selenide with silver ions diffused therein. Current methods provide silver ions within the germanium selenide material by initially depositing the germanium selenide glass layer onto a substrate, typically a first electrode, and then depositing a thin overlying layer of silver, typically by physical vapor deposition (i.e., sputtering). The thin silver layer is then exposed to electromagnetic energy such as ultraviolet (UV) radiation to diffuse silver into the germanium selenide layer, such that a homogenous distribution of silver throughout the layer is ultimately achieved. In an exemplary embodiment, the upper electrode is then formed from silver that is sputter deposited onto the metal-doped germanium selenide layer.
In the process of depositing silver to form the upper electrode, plasma generated during sputtering results in the generation of electromagnetic radiation, which drives additional silver into the metal-doped material. Although some doping of silver into the material is needed to provide a working device whereby silver from silver ions within the material plates out to grow the dendrite extension between the two electrodes, the amount of electromagnetic radiation generated during the sputtering process can drive excessive amounts of silver into the resistance variable material such that the device is rendered non-functional.
Therefore, a need exists for a process for fabricating memory cells comprising a resistance variable material that avoids such problems.